Earlier quoted context omitted.
They didn't go big enough: they should have become a payer/insurer. If they started with their ~1M US employees and then applied the flywheel to HC insurance, they could have essentially created a single-payer system by default. Slightly scary to think about Amazon having a near monopoly on healthcare, but ask yourself whether our current reality is better or worse…
Amazon is already an insurer, and has been for years. They are self-insured for the health plans they offer to employees, and only rely on third-party payers for network management and claims administration. They could take that part in house but it's a low-margin business where they would have no particular advantage.
If they cut out the middleman, they could negotiate directly with providers and Pharma, lowering the "fully loaded" cost of their payroll. It would be a massive savings.
Once they did it for 1M people, the hard work would be over, and they could sell Amazon plans to the public.